Hello Matt Holmes,

Our AC meter only read active power.

There are three kinds of power. Active Power,Reactive power and inspecting power.
Here we just talk about active power.
The calculating formula for active power is P=U*I*cosφ( cosφ represents power factor). As different load have different power factor, the current and active power is also different.
In general,we use P=UI, because the power factor of pure resistive load is closed to 1
The power factor of inductive and capacitive loads is between 0-1.
So when you test the pure resistive load, P is equal to or near U*I; but when you test the inductive and capacitive loads (e.g.television,computer), P< U*I.

Any more questions, please kindly let us know.

Best regards.